The prior art has sought for many years to provide useful systems and methods for nondestructive examination and testing of critical parts such as aircraft and automotive structural members, machine elements, architectural members, turbine blades, and the like. A wide variety of nondestructive examination methods have been developed. One of the more useful of these is eddy-current testing. Broadly stated, this method involves placement of an inductor, typically a coil of wire, into close physical juxtaposition to an electrically conductive member to be tested and energizing the coil with a sinusoid, typically at a frequency between a few Hz and 100 MHz. The magnetic field from the coil penetrates the mass of the member to some extent and induces an eddy current therein. By monitoring the response of the coil to the applied signal, an evaluation of the impedance of the overall system can be derived. If the coil is then moved a short distance with respect to the part to be inspected and the process repeated, a similar value can be detected. Eventually, the entire member can be thus examined. Discontinuities in the sequence of detected values which do not correspond to discontinuities intended by the designer of the part suggest that the part is flawed, i.e., includes a crack, a void, or the like.
The prior art shows a number of patents directed at coil structures for this and related uses. Flexible coils have been the subject of many patents and other publications, inasmuch as many such parts are not simple planar members. See, for example, Viertl et al U.S. Pat. Nos. 4,593,245 and 4,706,020 and Weatherly U.S. Pat. No. 4,639,708.
Conventionally, such coils are connected as part of a balancing circuit, such as a Wheatstone bridge circuit, which provides a very accurate and simple method for measuring a change in impedance of the coil. That is, the circuit is balanced or "nulled", after which any departure from the null position of the circuit can readily be measured. For example, in U.S. Pat. No. 4,107,605 to Hudgell, one or two sets of four coils, each of spiral shape, are connected in a bridge circuit and are provided on a flat or profiled substrate to conform to the surface of the member under test.
A balancing circuit for multiple-coil eddy-current probes is shown in U.S. Pat. No. 4,651,093 to Detriche et al. Other patents generally discussing eddy-current testing of workpieces include Baraona U.S. Pat. Nos. 4,543,528 and 3,886,793 to Cramer et al. Other circuits for use with eddy-current distance measuring detectors and the like are shown in Kawabata et al U.S. Pat. Nos. 4,288,747 and 4,042,876 to Visioli, Jr.
It is also an object of the art to form coils in a simple manner, desirably by printing conductors on a substrate, as opposed to winding wires on a bobbin or the like. See, for example, U.S. Pat. No. 4,301,821 to Frances, which shows spiral inductors printed on either side of a flexible substrate and connected by way of a through-hole at the center. Frances does not appear particularly to discuss the application of this coil to eddy-current examination of conductive members.
It wi11 be appreciated from the above that using a single-coil inductor to generate an eddy current in a workpiece for inspection purposes necessitates that the probe be physically scanned over the member to be examined. Obviously, it would be desirable to avoid this complexity. The Detriche patent discussed above shows a probe consisting of an array of discrete coils separately addressed by individual leads connected to a multiplexing unit to reduce the number of movements required. However, the resolution of this system, that is, its ability to provide an "image" of a flaw to be detected in a part, is limited by the number of coils in the probe assembly, which in turn is limited simply by the number of wires which can be conveniently connected between the probe and the multiplexer.
To address this limitation, the inventors and others have attempted to develop arrays of coils on a flexible substrate organized into rows and columns so as to be individually addressable, and so that the impedance signals detected by each coil can be compared to one another to locate defects in a part under the array. For example, in FIG. 2, there is shown a 60-element test probe 10 which was developed by the assignee of the application as part of a system described in "Flexible Substrate Eddy-Current Coil Arrays", a paper published in 1986 by one of the present inventors.
The prior art probe 10 consists of an array of six rows of coils 12, each row consisting of ten individual coils 12, each of which is generally as depicted in FIG. 3. The circuit of the probe 10 is shown in FIG. 4. One terminal of each of the coils 12 in each of the six rows is connected in common to a row addressing switch, and the second terminal of each of the coils 12 of each column is connected to a column addressing switch. When it is desired to examine a member underneath the substrate 18 on which the coils and the conductors interconnecting the coils are formed, a signal is applied between the appropriate one of the row leads indicated at 20 and the corresponding one of the column leads indicated generally at 22. Because the coils 12 in the rows and columns are effectively connected in parallel, there is substantial "leakage", such that the exciting signal does not drive only the particular coil of interest, but also all of the other coils on the array, to an extent which depends on the resistance of the individual coils and the number of coils.
More specifically, for the 60-element array of FIG. 2, it is found that the particular coil connected directly receives approximately 25% of the energy, while the remaining 75% is spread out among the other coils in the array, with none of them receiving more than on the order of 6% of the total energy. As the coil directly addressed receives four times as much energy as any of the other coils, the variation of its impedance signal is sufficiently stronger than that of any of the other coils so that the effective signal-to-noise ratio is not uselessly low.
However, if the number of elements in the array is increased, the difference between the amount of energy driving the coil at the junction between the row and column to which power is applied and that driving the other coils is reduced substantially. This limits the number of elements which can be usefully provided in an array as shown in FIGS. 2-4. As it is desired that the elements be closely spaced to provide resolution of correspondingly small defects in the part to be examined, the array of FIG. 2 is only capable of examining relatively small portions of members. Therefore, this device does not solve the problem of requiring physical scanning of the probe over the part to be inspected.
Of course, it would be possible to provide separate connections to each of the coils of the array, as in the Detriche patent discussed above. This would allow individual addressing of each coil. However, this would require an individual connection to each of the coils, which would lead to a prohibitively large number of connecting wires. For example, in the array of FIG. 2 having n rows and m columns, n+m conductors and a ground connection are required. If each coil of the array were to be separately connected, this would require (n.times.m)+1 conductors--an amount of conductors which would be very awkward to handle, even for a relatively small array such as shown in FIG. 2.
Most prior eddy-current inspection systems, including that of FIG. 2, have operated, as explained above, in the "impedance-mode", wherein the inductor used to induce an eddy current in the part to be inspected is also used to detect the response of the part; that is, the free-space impedance of the driver coil is effectively known, and changes in its impedance which occur when it is juxtaposed to a particular part to be inspected are measured. Sudden variations in the impedance are indicative of discontinuities in the part.
An alternative to impedance-mode eddy-current testing referred to as "reflectance-mode" eddy-current testing can also be performed, wherein a first conductor carries a current which induces an eddy current in the member, and a different conductor is used to measure the corresponding induced voltage. Again, the electromagnetic properties of successive portions of the part are successively measured, so that variations in these properties not corresponding to desired discontinuities in the part indicate flaws. More specifically, the art shows multi-conductor reflectance-mode arrays for sequentially examining portions of a member to be inspected without physically scanning the sensor over the part. Individual elements of an array of driver elements are successively driven with a suitable signal, and the induced voltages detected using an array of detector elements are compared to locate flaws.
For example, Chamuel U.S. Pat. No. 4,706,021 discloses a "crossed wire defect detector employing eddy currents". This consists, in a particularly pertinent embodiment shown in FIG. 2, of a first series of parallel wires 18 extending in a first direction and arranged to be successively connected to a source of energy 26 by a multiplexing switch 27. The other ends of the wires are connected together and grounded as shown. An orthogonally-arranged second array of parallel wires 24 is closely juxtaposed to but electrically insulated from the first set of wires. A first end of the second series of wires 24 is connected together and grounded, as shown, while a second multiplexing switch 29 is used to successively connect the wires 24 to processing electronics 31, which effectively compares the signals induced on the individual wires 24. A sudden change in the signal detected from one wire to the next can indicate the presence of a flaw 28 in a member 22 to be inspected.
One of the difficulties with the Chamuel approach is its inherent poor sensitivity to small defects. In order to complete the drive and pick-up circuits, the ground connectors shown must return to the processing electronics. The wires thus behave effectively as relatively large coils, that is, on the order of the overall array size. It is well known that the sensitivity of a coil to small defects varies indirectly with the coil size.
A second problem with this approach is the large and variable coupling between the drive lines and the pick-up lines. It will be appreciated by those of skill in the art, referring to FIG. 2, that the inductive coupling between the drive circuit and pick-up circuit depends directly upon the physical overlap between the drive circuit and the pick-up circuit. The voltage induced on the pick-up circuit when the drive voltage is connected to the upper line will be much larger when the processing circuit is connected to the left-most line than when processing circuit is connected to the right-most line. These "intrinsic" signal variations are much larger than those produced by small defects.
A third difficulty with the Chamuel approach is that, if a defect such as a crack or scratch extends in a direction parallel to the longitudinal extension of the driver wires 24, the defect may fail to be located. To this end, Chamuel proposes, with respect to FIG. 7, using two sets of detector wires orthogonal to one another to insure that no defect can be parallel to both. This only partially solves the problem. It would obviously be desired to eliminate this sensitivity to crack orientation.
It would also be desired to produce other improvements on the Chamuel system, specifically including simplified methods of manufacture, improved signal-to-noise ratio, and provision of reflectance-mode eddy-current testing in a flexible array, which can conform to contoured parts.
Another disclosure generally relevant to eddy-current techniques is U.S. Pat. No. 4,733,023 to Tomaru et al which shows two orthogonal sets of parallel wires under a plate which are successively interrogated to locate the presence of a magnetic member on the surface of the plate. This is not an eddy-current inspection device per se.